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The conventional ${p}$ -GaN in Schottky-type ${p}$ -GaN gate HEMTs is converted into an insulator-like ${p}$ -GaN, i.e., fully depleted ${p}$ -GaN in the reverse-biased Schottky junction under forward gate bias ( ${V}_{\text {GS}}$ ), by inadequate activation of Mg. Consequently, the gate current ( ${I}_{\text {G}}$ ) is reduced by 3–5 orders of magnitude at ${V}_{\text {GS}}$ of 7-V over a temperature range of 25 °C–150 °C, compared with devices adopting conventional adequate activation. The fully depleted ${p}$ -GaN converts the current-balance-based forward ${V}_{\text {GS}}$ division in the conventional partially depleted ${p}$ -GaN gate stack, into thickness-based voltage division in ${p}$ -GaN and AlGaN between gate and channel, which is like that in a MIS (metal-insulator-semiconductor) gate. As a result, the hole current through the Schottky junction is significantly reduced while the electron spillover current starts to dominate at high forward ${V}_{\text {GS}}$ . Besides, hot carriers are excluded from the forward gate breakdown mechanism.

Enhancement-mode (E-mode) submicron 0.45- $\mu \text{m}~{p}$ -GaN gate HEMTs on 200-mm high-resistivity-Si (HRS) substrates have been demonstrated with high uniformity and high overall combined performance for RF applications. The ${p}$ -GaN gate HEMT exhibits a positive threshold voltage ( ${V}_{\text {TH}}$ ) of +0.58 V at ${I}_{\text {D}}$ of 0.1 mA/mm, a peak transconductance ( ${G}_{\text {M}}$ ) of 150 mS/mm, high saturation current density of 630 mA/mm at a gate bias ( $V_{\text {GS}}$ ) of 6 V and >1 A/mm at a ${V}_{\text {GS}}$ of 11 V. A low OFF state leakage current of $1.5~\mu $ A/mm is obtained at a ${V}_{\text {DS}}$ of 48 V with a forward OFF-state breakdown voltage larger than 65 V. The device exhibits a current gain cut-off frequency ${f}_{\text {T}}$ of 22.4 GHz, and a power gain cut-off frequency ${f}_{\text {MAX}}$ of 45.3 GHz. At 5 GHz, large-signal load-pull measurement yields an output power density of 1.4 W/mm and a power added efficiency up to 55.4 % with a drain voltage less than 20 V.